MCAT Biology Question 77: Answer and Explanation
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Question: 77
2. Which of the following associations correctly pairs a stage of respiration with the muscle actions occurring during that stage?
- A. Inhalation—diaphragm relaxes
- B. Inhalation—internal intercostal muscles contract
- C. Exhalation—diaphragm contracts
- D. Exhalation—external intercostal muscles relax
Correct Answer: D
Explanation:
The muscles involved in ventilation are the diaphragm, which separates the thoracic cavity from the abdominal cavity, and the intercostal muscles between the ribs. During inhalation, the diaphragm contracts and flattens, while the external intercostal muscles contract, pulling the rib cage up and out. These actions cause an overall increase in the volume of the thoracic cavity. During exhalation, both the diaphragm and the external intercostals relax, causing a decrease in the volume of the thoracic cavity because of recoil of these tissues. In forced exhalation, the internal intercostals and abdominal muscles may contract to force out air. Thus, the only correct association from the given answers is choice (D).